There are many processes which require not just the measurement but also the controlled dispensing of small quantities of one or more materials into a larger amount of fluid or powder. It is usually convenient to add these materials as additive liquids because liquids are easier to dispense accurately than powder or other forms of solid material. For instance, in the manufacture of coated, dyed, printed, or painted, e.g. spray painted, materials, e.g. textiles by dip coating, spraying, printing etc., amounts of certain chemicals may be added to the bulk coloring material to provide additional functions, such as anti-fungus or anti-bacterial additives, antioxidants, stabilizers including UV-absorbers, surfactants, chemical or stain resistance ar water resistance (water proofing) agents, viscosity controllers, pH regulators, cross-linking agents or similar additives. Similarly, in the pharmaceutical or the food making industries micro-amounts of chemicals may be added to provide specific functionalities. Some of these additive liquids may be expensive or environmentally detrimental and therefore there is a general need to prevent excessive use or waste of these liquids Hence, it is advantageous to dispense the liquids as exactly as possible and also to provide means for reducing waste even in emergency conditions or during malfunction of the processing equipment. Further, in systems in which many liquids: are mixed together before processing, the mixed materials and partly processed materials often cannot be reused so that they must be disposed of and become waste. Hence, it is desirable to mix only small volumes of the materials together, so that on interruption of the process only small volumes of waste materials are involved. However, mixing small quantities together means that it is more difficult to maintain exact dosing as the absolute tolerances reduce in proportion to the amount mixed. For instance, when mixing colors, it is easier to maintain a stable color if larger quantities are mixed together and one batch of color is used for one production run. Further, if. very small quantities of liquids are dispensed, the flow rate of the liquid may drop below that which can be metered satisfactorily by commercially available metering systems such as metering valves. In general, commercially available metering systems become inaccurate at flow rates below 5 liters per hour.
CH-A-632 840 describes a device for controlling liquid flow which includes a measuring cylinder and a pump connected thereto. The height of the liquid in the cylinder is measured and the pump is controlled based on the calculated rate of change of height in the cylinder compared with a predetermined value. No indication is provided of how this device may be used in a paint, ink, dye or colored paste printer or sprayer or dyeing equipment.
EP-A-636 959 describes a hybrid analogue and binary flow control device in which an analogue fluid line containing a flow rate regulator is connected in parallel with binary fluid lines which each contain an on-off(binary) valve and a precision orifice. The binary lines have a sequential order of increasing predetermined flow rates. This known device is complex and requires very careful adjustment and maintenance. It is designed for laboratory instruments and is unsuitable for industrial processes.
GB-A-2 260 965 describes a metering and dispensing system for dispensing liquids with small flows. A pair of vertical metering tubes provided with level sensors are alternately filled and emptied. The sensors are spaced apart along each tube so as to indicate the passage of a known volume of liquid. The time taken for the liquid surface to move from one sensor to the other combined with the knowing volume expelled allows calculation of the flow rate of the liquid. Feed back control is provided so that if the time taken for the liquid to travel from one sensor to the other is too short or too long, the input pressure of the liquid to the tubes is reduced or increased respectively. One disadvantage of this system is that a knowledge of how much material has been dispensed can only be calculated discontinuously at the end of each emptying cycle. Such a system may be subject to hunting, i.e. that the feed back control system over compensates and under-compensates alternatingly so that the system never reaches stability and never gives a continuous flow. Further, the system cannot react to changes in flow during one cycle. The system reaction time can be reduced by reducing the volume in the tubes between the two level sensors. However, this has the disadvantage that the range of flow rates which can be dispensed accurately is reduced or that the same accuracy cannot be obtained over the whole range.
U.S. Pat. No. 4,906,165 describes a flow meter for use with a positive displacement pump or a gravity feed. The flow meter is connected to a positive head of liquid in order to charge a volume float chamber. The volume float chamber is a vertical tube. The volume float chamber is also connected to the suction side of a pump. Once the chamber is charged, the pump draws liquid from the chamber. Level sensors are used to measure the time taken for the liquid level in the chamber, as indicated by a float, to drop from a first to a second level and from this value the flow rate is calculated. During the time that the chamber is refilled the pump is switched off The average flow rate over one cycle is determined by extrapolating from the measured portion of flow to the complete time the liquid has been flowing. This known device does not control the flow continuously and suffers from having no feedback control.
U.S. Pat. No. 4,597,507 describes an apparatus for metering and feeding a solution which includes a container, an inlet valve for feeding solution into such container, and an outlet valve for allowing the solution fed into the container to flow out. A pressure transducer connected to the container measures the static head of the solution and outputs a signal proportional to the volume of solution in the container. A control device responsive to the output of the transducer closes the inlet valve when a predetermined volume of solution is reached and opens the outlet valve to allow gravity feed discharge of the solution. This known device has the disadvantage that the flow is discontinuous. The average flow rate is only adjustable by increasing or decreasing the delay between cycles resulting in a pulsed discharge. This reduce., the range of flow rates which can be dispensed effectively. The device works best when only a single flow rate is required. Further, as gravity feed is used, the range of viscosities of the solution and the output pressure of the known device is restricted.
Still another problem may be caused by non-linearities in the system. One source of non-linearity may be the liquid to be dispensed, e.g. if it is a thixotropic material or a material whose viscosity changes with the degree of sheer it has experienced. Other sources of non-linearity may be provided by the devices of the system such as pumps or valves with which the dispensed flow rate may depend upon the inlet or outlet pressure of the device or the difference between these two or on other factors. One approach to compensating for non-linearity is given in EP-A-403 280. In this known method the properties of the non-linear component (here, the liquid) are measured and recorded in a calibration phase. Tests are carried out over the likely useful range and a plurality of discrete points of differing flow values are measured and stored in a computer in the form of tables. During use the required value is looked up in the tables in the computer. If the required value lies between two previously calibrated values, the computer calculates some intermediate value in accordance with an interpolation routine. This known system has serious disadvantages when a plurality of liquids are to be dispensed. The number of points which have to be stored in the computer can be so great that the computer system becomes expensive and slow. Further, it is necessary to calibrate across the complete range of likely values even when some of these are never used later. This means that the system takes a long time to set up and is riot very flexible.
FR-A-2 611 059 describes a batch processor for dyeing textiles. The colored dye composition is created in a batch in a large vessel and is dispensed to a reaction chamber via a controllable valve which is controlled in accordance with the measured height of the dye composition in the vessel. If the process has to be stopped a large volume of colored dye left in the vessel must be disposed of
In the manufacture of carpet it is known to print the design onto the carpet rather than to weave it using different colored yarns. Such printing systems are many times faster than weaving. Such carpet printing systems dispense the color to the printing heads in the form of a paste. A high viscosity paste is necessary as there may be no movement of the color on the carpet after the printing process. Hence, the color paste must have sufficient stiffness that it does not migrate.
U.S. Pat. No. 4,403,866 describes a method of making paints in which a computer is used to control a multiplicity of metering pumps that are each individually connected to supplies of the paint components such as binder solution, solvent and colorants. The components are mixed and then circulated through a recycle loop in which a colorimeter checks the color value of the paint and feeds the results back to the computer. Any difference between the required and actual color is compensated for by the computer by adding more or less colorant. The procedure works best as a batch process, i.e. a quantity of paint is made and then checked. Adjustments are made until the required color is obtained and then the batch of paint is used. This known method has the disadvantage that after a machine malfunction, the large quantity of mixed paint in the mixing vessel may have to be discarded. Further, there are errors in colors which cannot be compensated for by the proposed technique without discarding some paint. e.g. if one of the color components is in a considerable excess it may be impossible to compensate for the error by the addition of other colors.
A batching method for paints is described in U.S. Pat. No. 4,705,083 which is supposed to achieve accuracy better that 1%. The method involves the operator in checking by hand the amount of any component delivered by a displacement pump driven by a stepping motor. Once the operator is satisfied that the correct flowv rate has been obtained, the machine is run. No method is described of how to maintain quality during a run.
Yet a further system of batching paint is known from EP-A-690 294 in which the quantity of paint of one color transferred to a mixing vessel is determined by a gravimetric scale supporting the mixing vessel. When a particular color is not being transferred to the mixing vessel, the paint is recirculated to prevent sedimentation.
It is an object of the present invention to provide an apparatus and a method of continuously, controllably and accurately dispensing liquids in an industrial apparatus for printing, dyeing, dip coating or spraying colors even at low flow rates, particularly to dispense liquids with a flow rate error of less than 5% at a flow rate as low as 0.33 ml per minute over long periods.
It is a further object of the present invention to provide an apparatus and a method for continuously, controllably and accurately dispensing liquids over a wide range of flow rates, preferably over a range of 1 to 100 times.
It is a further object of the present invention to provide an apparatus and a method for an industrial process, in particular printing of carpets, in which a plurality of liquids and optionally other flowable materials are mixed together continuously and accurately.